Two of the DLC missions for Mass Effect 2 (Lair of the Shadow Broker and Arrival) are important to the plot, but it's not like you won't understand what's going on in Mass Effect 3 if you skip them. Lair of the Shadow broker is one of my favorite missions in the game, though.
As long as you have From Ashes for Mass Effect 3, you're set. That's the one piece of DLC I'd really recommend not skipping.
But really, other than Pinnacle Station for the first game, most of the DLC has been pretty good, so it's worth getting if you end up enjoying the games.